DistrictHeating and Boilers on same HW loop causing temp issues

asked 2019-09-24 18:54:17 -0600

Abarker gravatar image

Is there any reason why having a DIstrictHeating object and Boiler:HotWater objects on the same hot water loop would cause issues with loop temps?

I have a DistrictHeating object and five Boiler:HotWater objects. I intend to have the district heating meet as much of the load as it can (it can't meet the peak load on its own), and then stage the boilers in sequential order to meet the remainder, so pretty straight forward. I have the district heating prioritized first on the plant equipment list. However when I model this for some reason my hot water loop temperature drops from a peak supply of 82C and a return of around 70C (when I don't have the DistrictHeating object modeled), down to supply and returns in the range of 22-25C when I do have it.

If I fake the district heating as a 6th boiler with its same capacity, efficiency of 1 and again with first priority, there are no issues and everything runs as expected with loop setpoints met. Other than changing the object type and corresponding branch, equipment list references to it, etc I've made no other changes. I prefer to avoid this however as I want to schedule the capacity of the district heating using a fraction multiplier.

What could cause this?

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